Energy storage systems known as “supercapacitors”, “ultracapacitors” or “electric double layer capacitors” (EDLCs) are composed of current collectors onto which a film of active material is applied. This system is then immersed in a solvent containing a salt and enables electrical energy to be stored for a subsequent use.
The active materials most used in double layer-type energy storage systems are activated carbons, due to their high specific surface area (generally in the range 500-2500 m2/g) and their relatively low cost. They are differentiated by their origin or precursor (hard coal, lignite, wood, fruit shells, etc.) and also by the type of activation that they have undergone, physical (that is to say with steam) or chemical (phosphoric acid, sodium hydroxide or potassium hydroxide, for example) and/or the type of purification post-treatment which gives them a set of characteristic properties.
The three main properties of the active materials, for example of the activated carbons, which are of interest for this application are the following:                a) the pore distribution, which determines the accessibility of electrolyte ions at the surface of the active material: the amount of ions accessible at the surface of the carbon determines the capacitance, expressed in farads (F) of the electrode, that is to say its energy density, whereas the mobility of the ions contributes to the surface resistance of the electrode, expressed in ohm.cm2 (Ω.cm2), itself inversely proportional to the power density. The pore distribution of activated carbons is generally described by:        the total pore volume, expressed for example in cm3 of nitrogen per gram of activated carbon;        the distribution, as a percentage, of this volume as a function of the size of the pores, which are classed as micropores (diameter <2 nm), mesopores (diameter 2-50 nm) or macropores (diameter >50 nm);        the specific surface area or BET surface area expressed in m2/g of activated carbon;        b) the purity: the behaviour of electrodes during ageing is determined, in particular, by the nature and amount of oxido-reducible impurities present in the activated carbon which prove damaging for the electrical properties of the carbon; and        c) the particle size distribution, which, in particular, influences the use of the carbon during the manufacture of the electrode.        
Certain types of activated carbons are known whose porosity makes it possible to obtain, more particularly, high energy densities or high power densities for the ultracapacitors.
In U.S. Pat. No. 5,430,606, activated carbons are described that are obtained by chemical activation using sodium hydroxide and washing with water; the energy storage systems manufactured with these carbons have good initial performances in terms of energy density, but no indication is supplied on their behaviour during ageing. Moreover, the production process described is expensive, as the activation is of chemical type.
U.S. Pat. Nos. 5,905,629, 6,060,424 and 5,926,361 describe ultracapacitors having a high energy density that are obtained from activated carbons having a particular porous structure composed mainly either of micropores (U.S. Pat. No. 5,905,629 and U.S. Pat. No. 6,060,424), or of mesopores (U.S. Pat. No. 5,926,361), but no indication is given on the behaviour during ageing of the electrodes, a fundamental property of ultracapacitors. Moreover, these carbons are obtained by an expensive process consisting of a phosphoric acid chemical activation of the activated carbon precursor followed by washing with water to remove the impurities and an additional heat treatment.
JP 09063907 describes the use in ultracapacitors of an activated carbon obtained by physical activation then washing with water and characterized by a size between 6 and 10 μm and a specific surface area between 1000 and 1500 m2/g. However, no indication is supplied on the impurities present in the final carbon nor on the actual ageing conditions to which the ultracapacitors have been subjected.
As is described in IEEE Spectrum from January 2005, page 29, the large-scale use of ultracapacitors, such as for example in the automotive industry, requires a 5-fold reduction in their cost and an increase in their energy storage capacity: none of the known technical solutions make it possible to obtain ultracapacitors that have, at the same time, energy and power densities that are high and stable over time, at a price compatible with the economic constraints of the automotive market.